Bifocal imaging optical system and imaging apparatus

Abstract

A configuration where a first group having a positive refractive power and a positive second group having a positive refractive power are placed sequentially from the object side is basically used. As the first group, a wide-angle first group and a telephoto first group are selectively switched over to be placed on the optical axis, thereby switching over the focal length between the wide-angle side and the telephoto side. Since the configuration in which the first group placed on the object side is switched is employed, adherence of dust or the like to the imaging surface due to the switching of the focal length can be prevented from occurring. Each group is configured by a minimum number of lenses. The whole system has a configuration which has a reduced number of lenses, and which is small and simple.

1. A bifocal imaging optical system comprising:
 a first group for wide angle imaging, which includes one lens and has a positive refractive power; 
 a first group for telephoto imaging, which includes one or two lenses and has a positive refractive power as a whole; and 
 a second group including one lens and having a positive refractive power, the second group being commonly used with both of the first group for wide angle imaging and the first group for telephoto imaging, wherein 
 each of the first group for wide angle imaging, the first group for telephoto imaging and the second group has at least one aspherical surface, 
 the first group for a wide angle and the first group for a telephoto are selectively used so as to switch an overall focal length of the bifocal imaging optical system between two focal lengths, wherein when a wide-angle imaging operation is performed, the first group for wide angle imaging and the second group are arranged on an optical axis sequentially from an object side of the bifocal imaging optical system, and when a telephoto imaging operation is performed, the first group for telephoto imaging and the second group are arranged on the optical axis sequentially from the object side, and 
 the bifocal imaging optical system satisfies conditional expressions:
   1.0 <f 1 w/fw <3.0  (1) 
   2.0 <f 2 /fw< 4.0  (2) 
   2.5 <f 1 t/fw <4.0  (3) 
 
 
     wherein
 fw is an overall focal length of the bifocal imaging optical system in a wide-angle side, 
 f 1 w is a focal length of the first group for wide angle imaging, 
 f 1 t is a focal length of the first group for telephoto imaging, and 
 f 2  is a focal length of the second group. 
 
   
   
     2. The bifocal imaging optical system according to  claim 1 , wherein
 the first group for wide angle imaging includes a meniscus lens having a positive refractive power and having a convex surface on the object side, 
 the first group for telephoto imaging includes at least one meniscus lens having a positive refractive power and having a convex surface on the object side, and 
 the second group includes a meniscus lens having a positive refractive power and having a convex surface on the object side in the vicinity of the optical axis. 
 
   
   
     3. The bifocal imaging optical system according to  claim 1 , wherein the second group is a plastic lens. 
   
   
     4. The bifocal imaging optical system according to  claim 2 , wherein the second group is a plastic lens. 
   
   
     5. An imaging apparatus comprising: a bifocal imaging optical system according to  claim 1 ; and an imaging device that outputs an imaging signal corresponding to an optical image formed by the bifocal imaging optical system.